Zhurong Observations Reveal Primary Evaporite Deposits in Mars Utopia Planitia

Mars has surprised scientists again. On a ~0.76-billion-year-old surface in southern Utopia Planitia, China’s Zhurong rover has detected a shallow horizon—only 5–10 centimeters thick—packed with platy rocks enriched in hydrous minerals. The finding points to water-related chemistry far later than many models had expected, raising fresh questions about when Mars could still host liquid environments capable of supporting life.

Until now, the exact mineral species behind these hydrous signatures remained uncertain, limiting researchers’ ability to reconstruct the planet’s recent water cycle. Different hydrated phases can form under very different conditions—especially whether they require prolonged liquid water, brief brines, or specific evaporative pathways.

In the new study, the team reports decimeter-scale crystal morphologies with a striking geometry: they resemble euhedral selenite, a distinctive crystal form of gypsum. Euhedral crystals grow from solutions that remain sufficiently stable for ions to organize themselves into ordered lattices, a clue that the rover’s target did not merely “weather” into place.

Why does that matter? Gypsum/selenite is interpreted here as a “primary evaporite,” meaning it precipitated directly from evaporating saline water at or near the surface, rather than being transported or produced by later alteration. The authors contrast this with sulfate deposits already reported on Mars, which may reflect different chemical histories.

Crucially, the paper argues that forming such primary evaporites requires surface water with a cumulative water-equivalent column of at least 6.25–25 meters over time. That estimate links mineral abundance and crystallization needs to plausible water budgets, not just to the presence of hydrated minerals.

But Mars did not have to be wet everywhere at once. The study suggests that either sustained or episodic upwelling of saline groundwater—potentially driven by magmatism—could have fed a very shallow, mostly ice-covered water body. At any particular moment, the actual liquid layer would likely have been about a meter or less.

If true, the setting becomes a chemical time capsule: brine would repeatedly infiltrate the near-surface ice-water interface, evaporate, and deposit new evaporites. Each crystal could therefore archive a sequence of geochemical conditions in the Amazonian period.

Perhaps most exciting, large selenite crystals may preserve fluid inclusions—microscopic pockets of ancient brine sealed within the growing mineral. Those inclusions could retain clues about the composition, salinity, and potential energy sources of Martian fluids, offering an unprecedented window into environments that might have been habitable, at least transiently.